FIELD OF THE INVENTION
The invention relates to a method for determining a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity. The invention also relates to a device for carrying out the method, having a computer unit which extrapolates from reference sound velocities to the mean sound velocity and is used to calculate the liquid level.
A particularly exact determination of the liquid level is required, for example, in the case of an underground dispensing system for fuel or generally in the case of a container for dangerous substances. In that case stringent environmental conditions require virtually any leakage to be detected and removed. In the case of a large tank with a cross-sectional area of 1 to 10 m.sup.2, even an erroneous measurement of the liquid level of only 1 mm leads to an erroneous determination of a liquid volume of 1 to 10 l.
A contactless ultrasonic measuring system which is known for the purpose of monitoring a fuel tank determines the liquid level with the aid of an echo time measurement of ultrasonic pulses, reflected at a liquid surface, in accordance with the echo sounding principle. It is known, moreover, to make use of a piezoceramic ultrasonic transducer which is suitable as a single component both for transmitting and for receiving ultrasonic pulses, for the purpose of measuring the echo time.
In order to calculate the filled level from the measured echo time, it is necessary to know the velocity of propagation of the ultrasonic pulses in the liquid. However, it is precisely in a large container such as a fuel dispensing system, that the density and the temperature in the liquid are not constant over the entire container volume. Rather, a density and temperature gradient forms in the vertical direction.
Since the sound velocity is a function of density and temperature, it is necessary to perform a correction of the sound velocity in order to determine the liquid level with a tolerance in the mm range. Again, no general specification of the sound velocity is possible for inhomogeneous liquids having a composition which fluctuates.
A solution which is known from U.S. Pat No. 4,748,846 is to measure the sound velocity with the aid of reference paths of known length directly in the liquid. For that purpose, additional reflectors for ultrasound are disposed at prescribed spacings in the vertical direction along the actual measuring path, along which an ultrasonic transducer emits. In order to calculate the liquid level, use is made of a mean sound velocity of liquid determined with the aid of the reflector respectively located closest to the liquid surface. That is an average sound velocity, since averaging is performed in the vertical direction over the height.
U.S. Pat. No. 3,394,589 also discloses the determination of a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity.
The accuracy achievable with the method becomes greater, with a smaller spacing between the successive reflectors.
However, as a disadvantage it is necessary to distinguish between reflections from the liquid surface and reflections from fixed reflectors, in a complicated signal detection method with the assistance of electronic masking times.
Limits to the achievable accuracy are therefore set. That holds even when reflectors are disposed separately next to the actual measuring path (U.S. Pat. No. 5,095,748), since even then the reflectors cannot be disposed at any desired density because of multiple reflections.